1. Field of the Invention
The present invention relates to a method of producing a semiconductor acceleration sensor or a semiconductor pressure sensor (hereinafter referred to as semiconductor dynamic sensor).
2. Description of the Related Art
A conventional method of producing a semiconductor dynamic sensor is illustrated in FIGS. 9 to 13.
Referring, first, to FIG. 9, a wafer 40 having an n-type epitaxial layer 42 is formed on a p-type substrate 41, a p.sup.+ -type diffusion layer 43 which is a piezo-resistance region is formed therein, and an n.sup.+ -type diffusion layer 44 is formed so as to be used as an electrode contact during the electrochemical etching. Then, a plasma nitride film (P-SiN) 45 is formed on the back surface of the wafer 40 and is patterned as desired by photo-etching.
Referring next to FIG. 10, the wafer 40 whose surface is protected with wax W is adhered to an alumina support substrate 46, and is dipped in an etching solution. An electric current is supplied to the n.sup.+ -type diffusion layer to effect the electrochemical etching in order to form lower isolation grooves 10 in the p-type substrate 41.
Then, with reference to FIG. 11, the nitride film 45 is removed, a resist 49 is applied onto the surface of the wafer 40 and is photo-patterned and, then, a resist 50 is applied onto the whole back surface of the wafer 40.
Then, as shown in FIG. 12, the epitaxial layer 42 is removed by etching through the opening of the resist film 49 thereby to form upper isolation grooves 51.
As shown in FIG. 13, next, the resists 49 and 50 are peeled off, and the wafer is cut.
In recent years, it has been urged to provide sensors having improved sensitivity in reduced sizes, and this can be effectively accomplished by reducing the thickness of a thin distortion-producing portion 52 where a piezo-resistance region 43 will be formed.
According to the above-mentioned method of producing a semiconductor dynamic sensor, however, the central portion of the wafer 40 must be vacuum-chucked onto a spinning table in order to spin-coat the surface of the epitaxial layer 42 with the resist 49 in FIG. 12 after the thickness of the epitaxial layer 42 has been partly reduced by etching the substrate 41 in FIG. 10. As the thickness of the epitaxial layer 42 is decreased, therefore, the thin portion of the epitaxial layer 42 tends to be broken by the vacuum pressure. According to the conventional production method, therefore, the thin distortion-producing portion 52 must have a minimum thickness (e.g., about several tens of microns) which withstands the vacuum chucking in the step of coating resist for photo-patterning after the substrate 41 has been etched. That is, it was not allowed to further improve the sensitivity by further decreasing the thickness of the thin distortion-producing portion 52.
As one application, the above-mentioned semiconductor dynamic sensor can be used as an acceleration sensor for automobiles. Acceleration sensors for automotive applications have been employing a semiconductor acceleration sensor made up of a piezo-resistance element (e.g., that disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2-231571). That is, a silicon chip is joined onto the seat, a moving portion of a beam structure is formed in a portion of the silicon chip, and a piezo-resistance layer is formed in the moving portion which has a thickness of about 40 .mu.m.
In this acceleration sensor, a thin beam is formed by subjecting the back surface of the silicon substrate to the etching with KOH until the thickness is decreased to a desired value. Then, by masking the beam portion, the front surface is subjected to the wet etching using a solution of the type of hydrofluoric acid and nitric acid (HF:HNO3:CH3COOH=1:2 to 8:0 to 8) in order to form a through hole and a beam structure.
However, when it is attempted to improve sensitivity of a general semiconductor acceleration sensor to sense small accelerations (0 to 1.5 G) so that it can be used as an acceleration sensor for ABS use, it becomes necessary to decrease the thickness of the thin portion (moving portion) of the silicon chip to be smaller than about 15 .mu.m. When the thin portion has a thickness which is as large as about 30 to 50 .mu.m, no problem arises even when the wet etching is effected with the solution of hydrofluoric acid and nitric acid to form a through hole and a beam structure since the thickness is sufficiently great. When the beam has a thickness of as small as 3 to 5 .mu.m, however, formation of the through hole by the wet etching with the solution of hydrofluoric acid and nitric acid makes it difficult to obtain strength to a sufficient degree.